In electrophotographic copying, an electrostatic latent image is developed on a primary image-forming member, such as a photoconductive surface, and is developed with a heat softenable thermoplastic toner powder to form a toner image. The toner image is thereafter transferred to a receiver, such as a sheet of paper, plastic or the like, and the toner image is subsequently fused to the receiver in a fusing station using heat, pressure, or both. A fusing station includes fuser members, which are typically rollers such as a pressure roller and a fuser roller, although fuser belts and the like may also be used. The essential function performed in the fusing station is the application of heat and pressure to the toner image on the receiver to fix the image to the receiver. In the discussion of this invention, the terms pressure roller and fuser roller will be used to refer to the two rollers used in the fusing station. It should be understood that reference to rollers also includes a reference to fuser belts and similar equipment that provide similar heat and pressure treatment for latent images on receiver sheets.
The fusing step is commonly carried out by passing the toner image-bearing receiver between a pair of engaged rollers that produce an area of pressure contact at the point of the engagement of the rollers known as a fusing nip. In order to form the nip, at least one of the rollers typically includes a compliant or conformable layer. Heat is transferred from at least one of the rollers to the toner in the fusing nip causing the toner to partially melt and attach to the receiver. In the case where the fuser member is a heated roller, a resilient and compliant pressure roller having a smooth surface is typically used. Where the member is in the form of a belt, such as a flexible and endless belt that passes around the heated roller, the belt typically has a smooth, hardened outer surface.
Most fusing stations, which are known as simplex fusers, attach toner to only one side of the receiver at a time. In such fusers it is common for a first one of the two rollers to be driven rotatably. The second roller is then rotatably driven by frictional contact with the first roller. Similarly, heat is typically applied to only one of the rollers, which roller is generally referred to as a fuser roller. The heat may be applied by the use of one or more heater rollers in contact with the heated roller to heat the exterior of the heated fuser roller or the heat may be applied internally to the heated fuser roller.
Various types of heater rollers have been used. The hardness of the rollers has been varied over wide limits in attempts to achieve better fusing results. Typically fuser rollers and pressure rollers may include a conformable layer that may be formed of any suitable material, such as polydimethylsiloxane elastomer.
Typically, fuser rollers include a hollow cylinder core, which is often metallic, with a roller cushion layer formed around the roller. Such cushion layers are commonly made of silcone rubbers or silcone polymers having a low surface energy such as polydimethylsiloxane, which minimizes adherence of toner to the roller, especially the heated roller. It is also known that cured polyfluorocarbon polymers and copolymers may be used to coat the cushion layer surface to further reduce the tendency of the toner to adhere to the roller and minimize contact of release oils with the cushion layer.
The cushion layer may include fillers including inorganic particles such as metals, metal oxides, metal hydroxides, metal salts, mixtures thereof and the like. The cushion layer may include a siloxane elastomer, such as a condensation cross-linked polydimethylsiloxane, which contains metal oxide fillers. Cushion materials of this type exhibit serious stability problems over time if they come in contact with release oils and the like.
Such materials are described more fully in U.S. Pat. No. 6,361,829 issued Mar. 26, 2002 to Jiann H. Chen, et al. (Chen '829) and assigned on its face to NexPress Solutions, LLC and U.S. Pat. No. 6,355,352 issued Mar. 12, 2002 to Jiann H. Chen, et al. (Chen '352). Both these references are hereby incorporated in their entirety by reference.
It is also known that various fluoropolymers, such as thermoplastic fluorocarbon polymers and random copolymers are useful as coatings on such rollers as release surfaces. Some such fluorocarbon thermoplastic polymers and thermoplastic random copolymers, including various additive materials, are disclosed in U.S. Pat. No. 6,372,833 issued Apr. 16, 2002 to Jiann H. Chen, et al. (Chen '833) and assigned on its face to NexPress Solutions LLC, and U.S. Pat. No. 6,429,249 issued Aug. 6, 2002 to Jiann H. Chen, et al. (Chen '249) and assigned on its face to NexPress Solutions, LLC. These patents are hereby incorporated in their entirety by reference.
While silicone rubbers and silcone polymers have been used widely as cushion layers, they have also in some instances been used as an exterior layer. Fluoroelastomers and rubbers, such as rubbers made of ethylene propylene diene monomers and the like have also been used as cushion layer materials. Unfortunately in many fusing processes the exterior of the fuser roller in direct contact with the toner, particularly a heated fuser roller, is coated with a release oil during fusing. Such release oils are generally detrimental to silicone rubbers and silicone polymers. Polyfluorocarbon polymers and random copolymers coated over the outside of the cushion layer have been found to be resistant to such oils and provide a low energy surface which readily releases from the toner and the receiver and are not adversely effected by the commonly used oils.
Notwithstanding the use of such materials for the production of rollers, it has been found in many instances the sheet and toner still tend to adhere to the fuser roller as the receiver sheet passes through and from the fusing nip. This creates a serious problem when the retention of the sheet is so serious as to result in wrapping the sheet around the roller and the like. Problems can also result when the release problems are less severe and the sheet does not release until forced to release by an air knife or the like. Such handling problems pose serious difficulties in routine operation of electrophotographic copying equipment. While various of the materials discussed above have been used in such applications, a need still exists for an improved process wherein improved releaseability of the receiver sheet and toner from the fuser roller as the sheet leaves the fusing nip is achieved. Continuing efforts have been directed toward the selection and the development of rollers that will meet this need and the development of improved release materials for use as external coatings for the pressure and fuser rollers.